[ARC5] Quick sensistivity comparison

[Kenneth G. Gordon]

On 13 Jun 2013 at 15:05, Geoff wrote:

> No surprise at all when you compare a low performance remote cutoff
> pentode with a sharp cutoff tube with 2X+ the transconductance.
> 
> If you want an apples to apples comparison then use a 12SG7 making
> socket changes as/if needed for them.

The 12SH7 has only a little greater transconductance than the 12SG7. Both 
are about double the 12SK7's 2300 micromhos: 4700 at max for the 12SG7 
and 4900 at max for the 12SH7. However, the 12SG7 is a remote cutoff type 
while the 12SH7 is a sharp cutoff type.

> I suspect there are other issues involved with that radio if you can
> hear a difference on an antenna that isnt a low noise type.

Ha! Low noise? You have got to be kidding: this antenna is so noisy it isn't 
even funny! It is a low (8 feet at the far end) end-fed "long" wire, about 50 
feet long, running east and west, connected to the receiver through a series 
of clip-leads that are draped over my computer and monitor. "Noisy" is too 
kind a word to describe it.

But, what is more important to me, what "other issues" might you suggest 
would be the cause of this observed reduction in noise?

And, BTW, disconnecting the HP-8640B connection to the receiver antenna 
post, yet leaving everything else as it is, results in absolutely nothing heard in 
the receiver until I have cranked the 8640B up about as far as it will go. 

Then, with the signal generator cable within an inch or so of the receiver 
antenna connector, I can JUST hear a signal.

Furthermore, when connected to the receiver, I can crank the 8640B's 
attenuator down one more "notch" and can hear nothing in the receiver 
either. I also have all the BNC caps on where they should be. All the original 
covers are on the signal genny and are all screwed down properly, and it is 
grounded. 

I really doubt if I am getting much leakage, but what do I know? I could 
definitely be wrong. I have been wrong before and will be again.

Cables (Pomona's) are all new, and the 50 ohm terminator is in a box. It is 
one of those which were ordinarily included with the URM-25 with 
male-BNCs on both ends.

Ken W7EKB